1. Field of the Invention
The present invention relates to a fresh water generator and a fresh water generating method for desalinating a target liquid containing residual chlorine by feeding the same to a fresh water generating cartridge having a reverse osmosis membrane.
2. Description of the Prior Art
When desalinating a target liquid (hereinafter referred to as raw water) such as service water or industrial water containing residual chlorine, a fresh water generator having a fresh water generating cartridge including a reverse osmosis membrane (RO membrane) separates the raw water into permeated water and concentrated water. In order to obtain permeated water in a necessary and sufficient flow rate in a fresh water generator having a limited scale, a permeated water storage tank is provided on the fresh water generator for storing unused permeated water therein or a pressure pump is provided on the fresh water generator for increasing the permeate flow rate per unit membrane area.
FIG. 8 is a block diagram showing an exemplary conventional fresh water generator having a pressure pump 100.
The fresh water generator shown in FIG. 8 employs the pressure pump 100 and a fresh water generating cartridge 4 for desalination. The fresh water generating cartridge 4 includes a reverse osmosis membrane. The reverse osmosis membrane is reduced in durability when exposed to chlorine for a long time, and hence an active carbon cartridge 2 is employed for pretreatment in order to remove residual chlorine contained in raw water.
The raw water is fed to the active carbon cartridge 2 through a raw water feed pipe 1. The pressure pump 100 feeds the water permeating the active carbon cartridge 2 as pretreated water to the fresh water generating cartridge 4 through a pretreated water feed pipe 3. The fresh water generating cartridge 4 having the reverse osmosis membrane separates the pretreated water into permeated water and concentrated water. The separated permeated water is taken out from the fresh water generating cartridge 4 as treated water through a permeated water outlet pipe 5. On the other hand, the concentrated water is discharged from the fresh water generating cartridge 4 through a concentrated water discharge pipe 7. The concentrated water discharge pipe 7 is provided with a pressure regulating valve 6, for suppressing the concentrate flow rate. Thus, permeation through the membrane is prompted in the fresh water generating cartridge 4.
However, the aforementioned fresh water generator requires a driver (not shown) such as a motor or an engine for driving the pressure pump 100 and a controller (not shown) for controlling the driver. Thus, the fresh water generating cost as well as the price of the fresh water generator increase. Further, noise generated in the pressure pump 100 and the driver results in a problem.
When the pretreated water is fed to the fresh water generating cartridge 4 without employing the pressure pump 100, a sufficient linear velocity cannot be obtained on the surface of the reverse osmosis membrane of the fresh water generating cartridge 4 and hence contaminants such as impurities adhere to the membrane surface. When such a fresh water generator is continuously operated for a long time, the performance of the reverse osmosis membrane of the fresh water generating cartridge 4 is reduced to disadvantageously deteriorate the quality of the obtained permeated water and reduce the permeate flow rate over time.
An object of the present invention is to provide a fresh water generator and a fresh water generating method which can be reliably operated at a low cost over a long time with no problem of noise.
The inventor has made various experiments and deep study for desalinating a target liquid without employing a pressure pump while suppressing propagation of bacteria, to find out that it is possible to desalinate a target liquid having a pressure of not more than 2 kgf/cm2 by feeding the same to a fresh water generating cartridge having a reverse osmosis membrane without employing a pressure pump when employing a reverse osmosis membrane having a rejection of at least 95% for an NaCl aqueous solution of 0.05% in concentration and a permeate flow rate of at least 0.1 m3/m2xc2x7dayxc2x7kgf/cm2.
The inventor has found out that a practical permeate flow rate can be obtained over a long time by providing a flushing device on a fresh water generator. The inventor has proposed the present invention on the basis of such recognition.
A fresh water generator according to an aspect of the present invention comprises a freshwater generating cartridge, including a reverse osmosis membrane, for desalinating a target liquid, a feed system feeding the target liquid having a prescribed pressure to the fresh water generating cartridge without through a step-up device stepping up the pressure of the target liquid and a flushing device flushing the reverse osmosis membrane of the fresh water generating cartridge.
The term xe2x80x9cflushingxe2x80x9d indicates an operation of washing the surface of a separation membrane with a water stream.
In this fresh water generator, the target liquid having a prescribed pressure is fed to the fresh water generating cartridge having the reverse osmosis membrane by the feed system and desalinated. The flushing device flushes the reverse osmosis membrane of the fresh water generating cartridge so that impurities remaining on the surface of the reverse osmosis membrane of the fresh water generating cartridge can be discharged from the fresh water generating cartridge.
In the fresh water generator, the target liquid having a prescribed pressure is fed to the fresh water generating cartridge in a non-powered manner without through a pressure pump serving as a step-up device, whereby neither a driver for driving the pressure pump nor a controller for controlling the driver is necessary. Thus, the fresh water generating cost is reduced while increase of the price of the fresh water generator is suppressed. Further, no problem of noise is caused by any pressure pump or driver. In addition, the fresh water generator requiring no pressure pump, no driver and no controller is miniaturized and no electric charge is required.
The prescribed pressure may be at least 0.3 kgf/cm2 and not more than 3 kgf/cm2. In this case, desalination can be performed without employing a pressure pump.
The flushing device may periodically perform a flushing operation. Thus, impurities remaining on the surface of the reverse osmosis membrane of the fresh water generating cartridge can be periodically discharged from the fresh water generating cartridge.
The flushing device may perform the flushing operation when fresh water generation work with the fresh water generating cartridge is started. Thus, impurities such as fungi generated and deposited in the fresh water generating cartridge during stoppage of fresh water generation work can be discharged from the fresh water generating cartridge.
The flushing device may perform the flushing operation during fresh water generation work with the fresh water generating cartridge. Thus, impurities remaining on the surface of the reverse osmosis membrane of the fresh water generating cartridge during fresh water generation work can be discharged from the fresh water generating cartridge. In this case, flushing may be periodically or non-periodically performed during fresh water generation work.
The flushing device may perform the flushing operation simultaneously with stoppage of fresh water generation work with the fresh water generating cartridge. Thus, impurities remaining on the surface of the reverse osmosis membrane of the fresh water generating cartridge during fresh water generation work can be discharged from the fresh water generating cartridge.
The flushing device may perform the flushing operation during stoppage of fresh water generation work with the fresh water generating cartridge. Thus, impurities such as fungi generated and deposited in the fresh water generating cartridge during stoppage of fresh water generation work can be discharged from the fresh water generating cartridge.
The flushing device may be a flushing valve stored in the fresh water generating cartridge. Alternatively, the flushing device may be a flushing valve provided in a concentrated liquid path of the fresh water generating cartridge. In this case, the concentrate flow rate is increased by opening the flushing valve to increase the linear velocity on the surface of the reverse osmosis membrane of the fresh water generating cartridge. Thus, impurities remaining on the surface of the reverse osmosis membrane are discharged from the fresh water generating cartridge.
The flushing device may include an automatic valve, and the fresh water generator may further comprise a controller controlling an opening and closing operation of the automatic valve. In this case, the controller automatically controls the opening and closing operation of the automatic valve.
The controller may control the automatic valve to open the automatic valve by a prescribed time at a prescribed time interval. Thus, the automatic valve is periodically and automatically opened for periodically flushing the fresh water generating cartridge. Consequently, impurities remaining on the surface of the reverse osmosis membrane are periodically discharged from the fresh water generating cartridge.
The ratio b/a of the flow rate b of the permeated liquid obtained from the fresh water generating cartridge to the flow rate a of the target liquid fed to the fresh water generating cartridge may be so set that b/a greater than 0.5.
In desalination with the fresh water generating cartridge having the reverse osmosis membrane, the target liquid fed to the fresh water generating cartridge is separated into a permeated liquid from which impurities are removed and a concentrated liquid in which impurities are concentrated. In order to increase the linear velocity on the surface of the reverse osmosis membrane for maintaining the membrane performance, the permeate flow rate b may be minimized so that the concentrate flow rate (a-b) approaches the target liquid flow rate a. When the ratio b/a of the permeate flow rate b to the target liquid flow rate a is minimized to 0.1 or 0.2, for example, the linear velocity on the surface of the reverse osmosis membrane is maintained in a state close to that on the inlet for the target liquid. However, the concentrated liquid is generally discarded and hence the flow rate b of the essentially required permeated liquid is uneconomically reduced if the ratio b/a of the permeate flow rate b to the target liquid flow rate a is at a low value.
In the fresh water generator according to the present invention, the membrane performance can be maintained by providing the flushing device, whereby the quality of the permeated liquid can be maintained even if the ratio b/a of the permeate flow rate b to the target liquid flow rate a exceeds 0.5. Consequently, an economical permeate flow rate can be obtained.
The fresh water generating cartridge may be formed by a plurality of fresh water generating cartridges connected in parallel and/or in series with each other. In this case, a large volume of permeated liquid can be obtained while preventing deterioration of the quality of the permeated liquid and reduction of the permeate flow rate.
If a plurality of fresh water generating cartridges are provided in parallel or in series with each other, the flushing valve may be stored in each fresh water generating cartridge or may be provided on the concentrated liquid path of each fresh water generating cartridge. Alternatively, the flushing valve may be provided on the rearmost one of connected portions between the concentrated liquid paths of the plurality of fresh water generating cartridges. Further, a prescribed number of fresh water generating cartridges arranged in parallel or in series with each other with a flushing valve provided on the rearmost one of connected portions between the concentrated liquid paths thereof may be integrated into a unit so that a plurality of such units are arranged in parallel or in series with each other.
The fresh water generator may be provided with a permeated liquid outlet path taking out a permeated liquid obtained from the fresh water generating cartridge and a permeated liquid discharge path discharging the permeated liquid. In this case, the permeated liquid taken out from the permeated liquid outlet path is employed for various applications as desalinated water. On the other hand, the permeated liquid discharged from the permeated liquid discharge path is not used but discharged as waste water.
In the fresh water generator thus provided with the permeated liquid discharge path, impurities deposited in the fresh water generating cartridge can be discharged from the fresh water generating cartridge through the permeated liquid discharge path along with the permeated liquid. The permeated liquid containing a large amount of impurities is discharged through the permeated liquid discharge path and hence only a permeated liquid having high quality is taken out from the permeated liquid outlet path. Therefore, the permeated liquid taken out from the permeated liquid outlet path is prevented from contamination with impurities and deterioration of quality.
The permeated liquid may be periodically discharged through the permeated liquid discharge path. Thus, impurities deposited in the fresh water generating cartridge can be periodically discharged from the fresh water generating cartridge through the permeated liquid discharge path along with the permeated liquid.
The permeated liquid may be discharged through the permeated liquid discharge path when fresh water generation work with the fresh water generating cartridge is started. Alternatively, the permeated liquid may be discharged through the permeated liquid discharge path during stoppage of fresh water generation work with the fresh water generating cartridge. In this case, impurities deposited in the fresh water generating cartridge during stoppage of fresh water generation work can be discharged from the fresh water generating cartridge through the permeated liquid discharge path.
The reverse osmosis membrane may have a performance of a rejection of at least 95% for an NaCl aqueous solution of 0.05% in concentration and a permeate flow rate of at least 0.1 m3/m2xc2x7dayxc2x7kgf/cm2. In this case, the target liquid having a prescribed pressure can be fed to the fresh water generating cartridge without employing a pressurizer for readily performing desalination.
The fresh water generator may further comprise a pretreater provided in the preceding stage to the fresh water generating cartridge for removing residual chlorine contained in the target liquid. In this case, the target liquid is fed to the pretreater in a non-powered manner without through a pressure pump serving as a step-up device, so that residual chlorine contained in the target liquid is removed. Thus, the reverse osmosis membrane of the fresh water generating cartridge is prevented from damage caused by residual chlorine contained in the target liquid, and durability of the reverse osmosis membrane can be improved.
A fresh water generating method according to another aspect of the present invention comprises steps of feeding a target liquid having a prescribed pressure to a fresh water generating cartridge including a reverse osmosis membrane without through a step-up device stepping up the pressure, and flushing the reverse osmosis membrane of the fresh water generating cartridge at prescribed timing.
In this fresh water generating method, the target liquid having a prescribed pressure is fed to the fresh water generating cartridge including the reverse osmosis membrane and desalinated.
In this fresh water generating method, the target liquid having a prescribed pressure is fed to the fresh water generating cartridge in a non-powered manner without employing a pressure pump as a step-up device, whereby neither a driver for driving the pressure pump nor a controller for controlling the driver is required. Therefore, the fresh water generating cost is reduced and increase of the price of a fresh water generator is suppressed. Further, no problem of noise is caused by any pressure pump or driver.
Further, impurities remaining on the surface of the reverse osmosis membrane of the fresh water generating cartridge can be discharged from the fresh water generating cartridge by flushing the reverse osmosis membrane of the fresh water generating cartridge at prescribed timing. Consequently, it is possible to prevent deterioration of the quality of a permeated liquid in a short period and reduction of the permeate flow rate over time caused by an insufficient linear velocity on the surface of the reverse osmosis membrane.
The fresh water generating method may further comprise a step of discharging the permeated liquid obtained from the fresh water generating cartridge through a permeated liquid discharge path at prescribed timing. In this case, impurities deposited in the fresh water generating cartridge and on the outlet for the permeated liquid can be discharged from the fresh water generating cartridge along with the permeated liquid. The permeated liquid containing a large amount of impurities is discharged through the permeated liquid discharge path, and hence no discharged impurities are mixed into the permeated liquid taken out from the permeated liquid outlet path. Therefore, the permeated liquid is prevented from deterioration of quality and a permeated liquid having high quality can be obtained.
The fresh water generating method may further comprise a step of applying the permeated liquid obtained from the fresh water generating cartridge to wash water, soft water for a boiler, soft water for preparing food, agricultural water for hydroponics, pure water for a laboratory, water for a humidifier or potable water.
The fresh water generating method may further comprise a step of feeding the permeated liquid obtained from the fresh water generating cartridge to an ion exchanger or a continuous electric regenerative ion exchanger. In this case, purity of the permeated liquid is further improved by the ion exchanger or the continuous electric regenerative ion exchanger, whereby extrapure water is obtained.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.